The Onset Temperatures of γ to α-Phase Transformation in Hot Deformed and Non-deformed Nb Micro-alloyed Steels

Yuan, Xiangqian; Liu, Z. Y.; Jiao, Sihai; Q., L.; Wang, G. D.

doi:10.2355/isijinternational.46.579

Cited by 36 publications

(25 citation statements)

References 29 publications

(39 reference statements)

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“…Similar expressions are also reported by Santos 11) who established a non-linear dependence between austenite grain size and ferrite start temperature (A r3 ), in which a ln(Dγ) expression was used. Yuan et al 12) also developed mathematical models corresponding to A r3 for Nb bearing steels with or without hot deformation. Continuous cooling transformation behavior was investigated via the thermal dilatation method, evaluating the influence of Nb content and deformation of austenite on phase transformation behavior.…”

Section: Transformation Start and Finish Temperaturesmentioning

confidence: 99%

Modeling of CCT Diagrams and Ferrite Grain Size Prediction in Low Carbon Nb–Mo Microalloyed Steels

Isasti¹,

García-Riesco²,

Jorge-Badiola³

et al. 2015

ISIJ Int.

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In this paper a multi-linear regression analysis is developed to predict continuous cooling (CCT) diagrams in low carbon Nb and Nb-Mo microalloyed steels. The inputs to the analysis include the weight percentage of alloying elements, the prior austenite grain size, the retained strain and the cooling rate. To develop the model, 11 steels with different combinations of Nb and Mo were considered. In some cases, the resulting equations have been validated with external data from the literature. Additionally, the model was also employed to predict hardness and ferrite grain size with the aim of providing a tool to link microstructural features with mechanical property predictions. Both Nb and Mo additions promote a reduction of ferrite and bainite start temperatures, where the effect is more pronounced for Nb in the bainitic region. Both microalloying elements contribute to an increase in hardness and a refinement of the microstructure.

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Section: Transformation Start and Finish Temperaturesmentioning

confidence: 99%

Modeling of CCT Diagrams and Ferrite Grain Size Prediction in Low Carbon Nb–Mo Microalloyed Steels

Isasti¹,

García-Riesco²,

Jorge-Badiola³

et al. 2015

ISIJ Int.

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“…And Ar 3 temperatures of thermal cycle TH0 and TH5, measured by dilatation test, are 610°C and 749°C, respectively ( 10) In the present work, the difference in Ar 3 temperature between two thermal cycles is also attributed to the difference in cooling rate of thermal history and grain size. The development of ductility trough is likely to be attributable to the formation of deformation induced ferrite.…”

Section: Hot Ductility Behaviormentioning

confidence: 62%

Assessment of Hot Ductility with Various Thermal Histories as an Alternative Method of in situ Solidification

et al. 2010

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“…Yuan et al 23) calculated the precipitation start times (t 0.05 ) of steel T1 by using the Dutta-Sellars model 24) and the method proposed by Jonas et al 25) The results showed that precipitation could hardly occur before transformation in non-deformed steel T1, while coarse Nb precipitates were likely to form in hot deformed steel T1 before ferrite transformation started. Figure 7(a) shows that precipitates in the size of 30-50 nm (Arrow A) formed in hot deformed steel T1.…”

Section: Precipitates Prior To Ferrite Transformationmentioning

confidence: 99%

Effects of Nano Precipitates in Austenite on Ferrite Transformation Start Temperature during Continuous Cooling in Nb–Ti Micro-alloyed Steels

Yuan¹,

Liu²,

Jiao

et al. 2007

ISIJ Int.

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In the present paper, transformation behavior during continuous cooling in non-deformed and hot deformed Nb-Ti micro-alloyed steels was investigated by using the thermal dilation method. Ti content in Nb-Ti bearing steels varied from 0 to 0.031 mass% with Nb content being kept to be constant. Thermal dilation curves were measured at different cooling rates, from which continuous cooling transformation (CCT) curves were built up. For non-deformed Nb-Ti steels, it was observed that ferrite transformation start temperatures (A r3 ) decreased with increasing Ti content up to 0.015 mass%, leveled off in the range of 0.015 to 0.027 mass% Ti, and drastically decreased thereafter. For hot deformed Nb-Ti steels, A r3 temperature did not exhibit significant difference with Ti addition lower than 0.027 mass%, and decreased drastically by further increasing Ti content. Austenite grain size (D g ), Nb-Ti precipitates and residual strain were taken into account to explain the variation of A r3 temperatures. Based on the experimental results, mathematical models for the calculation of A r3 for non-deformed and hot deformed Nb-Ti micro-alloyed steels were developed.KEY WORDS: Nb-Ti micro-alloyed; CCT curve; A r3 temperature; transformation; nano precipitate.across the specimen center, where the thermocouple was welded. Fernández et al. 20) calculated the amount of Nb and Ti precipitated at different temperatures for either Nb or Nb-Ti micro-alloyed steel, which showed that a reheating temperature of 1 220°C was proper for the steels used in this work. Figure 1 shows the schematic drawing for the thermo-mechanical treatment. The reheating temperature for these steels was chosen to be 1 220°C and held for 5 min to be fully austenitized. After reheating, the specimens were cooled down to 900°C at a cooling rate of 5°C/s, being held for 20 s and cooled down to 200°C at different cooling rates from 0.5 to 30°C/s. To study the effect of hot deformation on phase transformation behavior in the steels, a compression with a true strain of 0.693 was applied to the specimen at a strain rate of 5 s Ϫ1 after being held for 10 s at 900°C, with the laser beam being able to follow the welding spot on the specimen center surface during the deformation.After thermo-mechanical treatments, all specimens were cut in the middle along longitudinal direction across the thermocouple welding points and prepared for optical metallographic examination in the conventional way to observe the transformed microstructure by using an image analysis system, CCT curves were drawn by the temperatures determined on the dilatometric curves with the help of optical metallographic observations.To measure the austenite grain sizes prior to transformation, some of the samples were quenched from temperatures just above A r3 determined by the dilatometric tests. Carbon extraction replicas from the quenched samples, which were continuously cooled to A r3 at a rate of 1°C/s before quenching, were prepared to observe the precipitates prior to transformation by TEM (transmis...

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The Onset Temperatures of γ to α-Phase Transformation in Hot Deformed and Non-deformed Nb Micro-alloyed Steels

Cited by 36 publications

References 29 publications

Modeling of CCT Diagrams and Ferrite Grain Size Prediction in Low Carbon Nb–Mo Microalloyed Steels

Modeling of CCT Diagrams and Ferrite Grain Size Prediction in Low Carbon Nb–Mo Microalloyed Steels

Assessment of Hot Ductility with Various Thermal Histories as an Alternative Method of in situ Solidification

Effects of Nano Precipitates in Austenite on Ferrite Transformation Start Temperature during Continuous Cooling in Nb–Ti Micro-alloyed Steels

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